linux-kernel - [PATCH 1/3] cgroup/cpuset: Use static_branch_enable_cpuslocked() on cpusets_insane_config

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Message-ID: <20250806172430.1155133-2-longman@redhat.com>
Date: Wed,  6 Aug 2025 13:24:28 -0400
From: Waiman Long <longman@...hat.com>
To: Tejun Heo <tj@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Koutný <mkoutny@...e.com>
Cc: cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Waiman Long <longman@...hat.com>
Subject: [PATCH 1/3] cgroup/cpuset: Use static_branch_enable_cpuslocked() on cpusets_insane_config_key

The following lockdep splat was observed.

[  812.359086] ============================================
[  812.359089] WARNING: possible recursive locking detected
[  812.359097] --------------------------------------------
[  812.359100] runtest.sh/30042 is trying to acquire lock:
[  812.359105] ffffffffa7f27420 (cpu_hotplug_lock){++++}-{0:0}, at: static_key_enable+0xe/0x20
[  812.359131]
[  812.359131] but task is already holding lock:
[  812.359134] ffffffffa7f27420 (cpu_hotplug_lock){++++}-{0:0}, at: cpuset_write_resmask+0x98/0xa70
     :
[  812.359267] Call Trace:
[  812.359272]  <TASK>
[  812.359367]  cpus_read_lock+0x3c/0xe0
[  812.359382]  static_key_enable+0xe/0x20
[  812.359389]  check_insane_mems_config.part.0+0x11/0x30
[  812.359398]  cpuset_write_resmask+0x9f2/0xa70
[  812.359411]  cgroup_file_write+0x1c7/0x660
[  812.359467]  kernfs_fop_write_iter+0x358/0x530
[  812.359479]  vfs_write+0xabe/0x1250
[  812.359529]  ksys_write+0xf9/0x1d0
[  812.359558]  do_syscall_64+0x5f/0xe0

Since commit d74b27d63a8b ("cgroup/cpuset: Change cpuset_rwsem
and hotplug lock order"), the ordering of cpu hotplug lock
and cpuset_mutex had been reversed. That patch correctly
used the cpuslocked version of the static branch API to enable
cpusets_pre_enable_key and cpusets_enabled_key, but it didn't do the
same for cpusets_insane_config_key.

The cpusets_insane_config_key can be enabled in the
check_insane_mems_config() which is called from update_nodemask()
or cpuset_hotplug_update_tasks() with both cpu hotplug lock and
cpuset_mutex held. Deadlock can happen with a pending hotplug event that
tries to acquire the cpu hotplug write lock which will block further
cpus_read_lock() attempt from check_insane_mems_config(). Fix that by
switching to use static_branch_enable_cpuslocked().

Fixes: d74b27d63a8b ("cgroup/cpuset: Change cpuset_rwsem and hotplug lock order")
Signed-off-by: Waiman Long <longman@...hat.com>
---
 kernel/cgroup/cpuset.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index f74d04429a29..bf149246e001 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -280,7 +280,7 @@ static inline void check_insane_mems_config(nodemask_t *nodes)
 {
 	if (!cpusets_insane_config() &&
 		movable_only_nodes(nodes)) {
-		static_branch_enable(&cpusets_insane_config_key);
+		static_branch_enable_cpuslocked(&cpusets_insane_config_key);
 		pr_info("Unsupported (movable nodes only) cpuset configuration detected (nmask=%*pbl)!\n"
 			"Cpuset allocations might fail even with a lot of memory available.\n",
 			nodemask_pr_args(nodes));
-- 
2.50.0